Tag: galaxy cluster

It’s generally said that discoveries in science tend to be at the thin hairy edge of what you can do — always at the faintest limits you can see, the furthest reaches, the lowest signals. That can be trivially true because stuff that’s easy to find has already been discovered. But many times, when you’re looking farther and fainter than you ever have, you find things that really are new… and can (maybe!) be a problem for existing models of how the Universe behaves.

Astronomers ran across just such thing recently. Hubble observations of a distant galaxy cluster revealed an arc of light above it. That’s actually the distorted image of a more distant galaxy, and it’s a common enough sight near foreground clusters. But the thing is, that galaxy shouldn’t be there.

This picture is a combination of two images taken in the near-infrared using Hubble. The cluster is the clump of fuzzy blobs in the center left. The small square outlines the arc, and the big square zooms in on it.

The cluster is unusual. It’s at a distance of nearly 10 billion light years away. Clusters have been seen that far away, so by itself that’s not so odd. The thing is, it’s a whopper: the total mass in all those galaxies combined may be as much as a staggering 500 trillion times the mass of the Sun, making this by far the most massive cluster seen at that distance.

But that arc… First, things like this are seen pretty often near clusters. They’re gravitational lenses: the gravity from the cluster bends the light from a more distant galaxy in the background, bending its shape into an arc. See Related Posts below for lots of info and cool pictures on these arcs. In this case, I’ll note the shape of the arc implies the biggest galaxy in the cluster, the one right below the small square, is doing most of the lensing.

But here’s the problem: the galaxy whose light is getting bent has to be on the other side of the cluster, and that cluster is really far away. Note only that, the galaxy has to be bright enough that we can see it at all. Combined, this should make an arc like this rare. Really rare.

So rare, in fact, that it shouldn’t be there at all! The astronomers who did this research worked through the physics and statistics, and what they found is that the odds of seeing this arc in this way are zero. As in, what the heck is it doing there at all?

Galaxy clusters are collections of galaxies held together by their own gravity. We see clusters all over the place, and they’re among the largest structures in the Universe. We can find them at large distances, which means we see them as they (and the Universe) were young — it takes light a long time to travel across the cosmos. Astronomers went looking to find extremely distant clusters of galaxies, and found one at a staggering distance: 12.7 billion light years away!

Here’s an image showing the central part of the cluster:

[Click to bigbangenate.]

Each of those circled red dots is a young galaxy, so distant that the light has been on its way here for more than 90% of the current age of the Universe! And they’re almost lost among all those other stars and galaxies in the image (though their intense red color helps… as to why they’re red, read on).

Finding this cluster was a magnificent achievement. The astronomers used the massive 8.2 meter Subaru telescope to look at large swaths of the sky. They looked at the colors of the galaxies they found (PDF); distant objects would be so far away their light is significantly redshifted by the expansion of the Universe itself (I explain how this works here and here).

Galaxies are distributed throughout space, so you expect to see them scattered across the sky as well as in redshift (distance). When looking at one part of the sky, however, they found an unusually high concentration of galaxies that were very red. Using a different camera on Subaru, they took spectra of those galaxies — breaking the light up into very fine divisions of colors, like a rainbow with hundreds of colors in it — to accurately measure the redshifts of those galaxies. Spectroscopy of objects that faint is no easy task, but Subaru is a big ‘scope, and collect a lot of light even from faint objects at the remote reaches of the Universe,

The astronomers confirmed that many of the galaxies in their sample were at the same redshift (z = 6 for those in the know — which is a mighty big redshift). The odds of these galaxies all being at the same distance happening by chance is extremely small: only about one in a billion! So it’s pretty clear these galaxies really are physically associated with each other.

That is, clustered together.

This makes the cluster the most distant ever found that has been confirmed spectroscopically — one other has been found that might be farther away, but it hasn’t been confirmed yet. At 12.7 billion light years away, that means we see this cluster as it was a mere one billion years after the Universe itself formed! That provides key information about conditions in the early Universe, which are critical to understanding how it formed and changed as it aged.

The cluster itself is vast — it’s something like 50 million light years across. The team of astronomers used various methods to determine its mass, and their best guess is that its total mass is several thousand times the mass of our entire Milky Way galaxy! The estimation methods they used are fairly fuzzy, so it’s not clear how accurate this number really is. Still, the cluster is clearly huge, and massive. If we could see it today, it would probably rank among the largest structures in the Universe.

That’s not terribly surprising, if you think about it: only the biggest monster clusters can be seen at such a mind-crushing distance. The smaller ones will be harder to detect, so we’re likely to find the biggest.

Still, holy cow. I have read and written about extremely distant objects many, many times over the years, and have no doubt: I get chills every single time I think about this stuff. It wasn’t that long ago when the entire human race couldn’t be bothered to look beyond the tip of its collective nose. Now we can look into the fires of the Universe’s birth, into that forge itself, and tease out the secrets of how we came to be.

[The Desktop Project is my way of clearing all the pretty pictures off my computer’s desktop, by posting one per day until they’re gone. I think this week is it – I’m almost out!]

Dark matter is funny stuff. We’ve known about its existence for many decades, and the more we look the better our evidence gets. We know it has mass, and therefore gravity, but we don’t know what it is! We do, however, know what it isn’t: normal matter of any kind, like cold gas, rogue planets, black holes, dead stars, or anything else made of protons, neutrons and the other types of particles we deal with in everyday life.

Since careful observations have shown clearly it can’t be any kind of normal matter, it therefore must be some sort of exotic flavor of matter, some kind of particle we haven’t yet seen.

One thing we’re pretty sure about it, though, is that it doesn’t interact with normal matter except through gravity. Dark matter can pass right through you and you’d never know it. But put enough of it in one spot, and its gravity will reveal its presence.

Which is why the galaxy cluster Abell 520 is such a mystery. Here’s the beauty shot:

Pretty, isn’t it? Abell 520 is a galaxy cluster about 2.4 billion light years away, and a mass of several trillion times our Sun’s — it’s made of galaxies, each with billions of stars in them. And a galaxy cluster is a collection of hundreds or even thousands of galaxies bound together by their gravity. In fact, Abell 520 is more than one cluster: it’s actually a collision between two or more clusters! As they move through space, clusters can collide, and actually quite a few of these cosmic train wrecks are known.

When clusters collide, a lot of things happen. The gas clouds in between galaxies in the two cluster slams into each other, heating up to millions of degrees and glowing in X-rays. In the picture above, that gas has been colored green so you can see it (invisible to the eye, the X-rays were detected by the Chandra Observatory). The orange glow is from stars in galaxies (as seen by the Canada-France-Hawaii and Subaru telescopes). The blue is actually a map of the dark matter made using Hubble observations. The gravity of dark matter distorts the light passing through from more distant galaxies, making it possible to map out the location of the otherwise invisible stuff (you can read about how that’s done here and here).

Since dark matter doesn’t interact with normal matter, we expect it to simply pass through the collision point, sailing on as if nothing had happened. That’s been seen in a half dozen other galaxy cluster collisions, including the Bullet Cluster — hailed as definitive proof of the existence of dark matter — as well as Abell 2744 aka Pandora’s cluster (seen here on the right), and the newly found Musketball cluster.

But Abell 520 isn’t like those others. The problem is, there’s a clear peak in the dark matter right in the middle of the cluster, not off to the sides as you might expect. It looks as if the dark matter slammed to halt in the middle of the collision instead of sailing on.

Here’s the thing: this does not mean dark matter doesn’t exist, or we’re wrong about it. The other clusters I mentioned above make it clear we do have a pretty good grip — so to speak — on the behavior of dark matter.

We live in the Milky Way galaxy, a collection of more than a hundred billion stars forming a flat, spiral disk. Our galaxy is in turn part of a small group called the Local Group, just a few dozen members strong, of which we are among the largest. But galaxies live in larger groups yet, called clusters. Some have hundreds of galaxies, and some thousands. In the direction of the constellation of Hercules is one such smaller cluster, called (duh) the Hercules Cluster, just under 500 million light years from Earth. The VLT Survey Telescope took a look at the cluster and produced this spectacular picture of it:

[Click to galactinate, and you want to; I reduced the size considerably to fit it here.]

The cluster is unusually rich in spiral galaxies, and unlike bigger groups doesn’t have one, massive galaxy sitting at its core (the result of a bigger galaxy falling to the center and eating lots of other galaxies, growing huge in the process). Still, the small size of the cluster means a lot of its members are interacting, and if you look closely you see lots of them tugging at the others:

That edge-on spiral in the lower right is clearly warped, so I expect it’s suffered a near miss from another galaxy in the past few million years (maybe that little spiral above it, or more likely the severely messed-up fuzzball to the left), and other examples aren’t hard to find.

As an aside, when I was poking around the big image I saw lots of red dots aligned next to green ones on the left near the bottom, and realized that must be an asteroid, captured as it moved slowly across the field of view in the multiple exposures and different filters used to make this picture. A long green streak below that may be another asteroid moving much rapidly, or possibly a satellite that streaked across one exposure.

Take a look for yourself. What do you see?

And a thought for you: This small cluster is part of a larger complex called the Hercules supercluster, made up of many smaller groups like the Hercules cluster. Altogether, the supercluster is something like 300 million light years across… and is still not the largest structure. Hercules, together with the Coma Cluster and Leo Cluster, comprise what’s called the Great Wall: a vast structure that is among the largest in the Universe — it’s so big that even at its distance of several hundred million light years away it spreads across more than one-third of the visible sky!

Thinking about these types of things can numb the mind… but remember, the most amazing thing to me about all of this is that we can know them at all. We’re a part of all this, and when we look out at it, when we examine it, we are learning about ourselves. I think peering out into the cosmos so that we can better understand ourselves is one of the noblest things we humans can do, and using science as our tool the best way to do it.

And look what’s it’s given us! The entire Universe! We cannot possibly ask for anything more.

Sometimes, I like to think of a photon of light as a car on a road. As the road dips and curves, a car has to follow that path, dipping and curving as well. It might be weird to think of space as curving, but it does. Gravity from massive objects warps space, and a beam of light moving through that curved space curves along with it.

This is the principle behind what’s called gravitational lensing. A beam of light passing by an object — a big galaxy, say, or a cluster of galaxies — bends one way. A beam headed in a slightly different direction bends a slightly different way. This can really mess with what we see… which I can prove! Check this out: a Hubble image of the galaxy RCSGA 032727-13260.

What a mess! All those arcs and blue smudges are images of that one galaxy. The light from that galaxy traveled nearly 10 billion light years to get here! But when it was halfway here, that light passed by the big cluster of galaxies — the red fuzzballs — in the middle of the image. As it did, the curvature of space distorted and warped the light from the galaxy, and by the time it reached us here at Earth the image looks like this. The outstretched, smeared-out arc is amazing; I’ve never seen one that long and well-defined before.

Not only that, but the image gets broken up into several separate images. There are no fewer than four different repetitions of the background galaxy in the big image. To show that, I put three of them together here. It’s goofed up, to be sure, but you can kinda sorta see they are the same galaxy, flipped over and/or smudged out.

The cool thing about this is we can learn about the more distant galaxy by examining these images. Read More

In what has become an annual tradition here at BA Central, literally the day I post my gallery of best pictures of the year, something comes along that really would’ve made it in had I seen it even a few hours earlier. In this case, it’s a combined Chandra X-Ray Observatory and optical Very Large Telescope image of galaxy clusters colliding that’s so weird that at first I thought for sure it was Photoshopped! But it’s real, so check this out:

What you’re looking at is a collision on a massive scale: not just two galaxies, but two clusters of galaxies slamming into each other, forming this object, called Abell 2052. The total mass of this combined cluster is almost beyond imagining: something like a quadrillion times the mass of the Sun — 1,000,000,000,000,000 Suns! Note that our galaxy has about a hundred billion stars in it, so Abell 2052 is about 10,000 more massive. Yikes.

Like I said, insanely cool. The cluster has thousands of galaxies in it, and a total mass of something like a quadrillion — that’s 1,000,000,000,000,000 — times the mass of our Sun!

The image was taken as part of a program called CLASH, for Cluster Lensing And Supernova survey with Hubble. A large group of astronomers from ten different countries are observing more than two dozen such distant clusters to look for many interesting things, including exploding stars (which help us gauge the expansion rate of the Universe), very distant galaxies (to help us understand the early Universe), and to look for dark matter.

Dark matter is stuff that doesn’t emit light, but has mass. Careful observations over the years have ruled out pretty much every form of normal matter we can think of, from simple hydrogen clouds to black holes. Whatever this stuff is, it’s weird, not matter as we know it.

But we do know it’s there. Its gravity affects how spiral galaxies rotate, how clusters like MACS 1206 stay together, and can even bend light from more distant galaxies as it passes through. That last bit is the big deal here.

Astronomers have found a bloated, massive galaxy that may be a record-breaker: the most massive galaxy in the near Universe. The mass isn’t exactly clear, but it may be 13 trillion times the mass of the Sun!* That’s easily twenty times the mass of the Milky Way!

That’s an image from the 8-meter Gemini South telescope in Chile, and it shows the cluster Abell 3827, a 1.4 billion-light-year-distant collection of hundreds of galaxies all bound together by their own gravity. It’s a pretty rich cluster as they go. Many like it have one big galaxy in the core, called the central dominant galaxy (or sometimes cD for short), and it’s usually a few times bigger than any other galaxy in the cluster.

In the case of Abell 3827, though, the cD — called ESO 146-IG 005 — is out of control. The Milky Way is considered a big galaxy, and has maybe 400 billion times the mass of the Sun in total, but 146-IG is hugely bigger, swollen and ginormous. It’s far more massive than any other galaxy we’ve seen out to that distance. That glow you see in the center of the cluster is just from 146-IG all by its lone self, and it dominates the entire core of the cluster.

Astronomers have found the most distant galaxy cluster ever seen: the sexily-named SXDF-XCLJ0218-0510.

First, the picture, then the words:

Yikes! What’s all that then?

Okay, first, this picture is littered with stars and galaxies. The galaxies are so far away they’re hard to distinguish from the stars! The dots that have arrows pointing to them are the galaxies that are most likely part of the cluster. The ones with circles have had their distance measured and are known to be part of the cluster for sure. The contour lines represent the detection of very hot gas, which is a dead giveaway that we’re dealing with a cluster here; all big clusters have gas swirling around them that gives off X-rays; the lines are like a topographic map telling you where the (otherwise invisible) gas is in the picture.

"So what?", you might say. We’ve seen lots of clusters before. Ah, but this one is different: it’s a whopping 9.6 billion light years away.